To assure accurate position of an image on a sheet of paper in a copier or page printer using cut sheets, it is important that the sheet have reasonably accurate cross-track registration when receiving that image. The term "cross-track" is used to describe the direction across the path of travel of a sheet, and "cross-track registration" would be accurate location of that sheet in the cross-track direction. "In-track" is the direction parallel to the movement of the sheet in its path and the in-track edges of a sheet would be the edges parallel to the movement of the sheet.
U.S. Pat. No. 4,685,664, Petersdorf, shows a cross-track registration device for a sheet transport system in which upstream and downstream pairs of rollers drive a sheet along a path. As the sheet approaches the downstream pair of rollers while being driven by the upstream pair of rollers, the downstream pair of rollers is stopped. As the leading edge of the sheet engages the stopped downstream pair of rollers a loop is formed in the sheet. After formation of the loop the downstream pair of rollers is driven to advance the sheet. The loop is maintained by the continuously driven upstream rollers. A sensing device senses the cross-track position of an in-track edge of the sheet after it exits the downstream pair of rollers. The downstream pair of rollers are moved axially until the in-track edge being sensed is located at a predetermined cross-track position as sensed by the sensing means to thereby register the sheet in the cross-track direction. Cross-track movement of the downstream rollers without tearing the sheet is permitting by the loop.
This device is very effective in registering a sheet in the cross-track direction, however, the requirement that the downstream pair of rollers be stopped during a portion of each sheet's transportation through the device requires complicated and expensive timing mechanism for the drives on the downstream pair of rollers.